And I'm Robert Siegel. Today, in the trial of George Zimmerman, a key witness bolstered Zimmerman's claim that he acted in self-defense when he killed teenager Trayvon Martin. The witness was a neighbor in the Sanford, Florida community where Zimmerman encountered Martin and he was the only person to see them fight before Zimmerman fired the gunshot that ended Martin's life.

Here's NPR's Greg Allen.

GREG ALLEN, BYLINE: On that night in February of last year, John Good and his wife were watching TV in their townhome when Good says he heard sounds outside. In the courtroom in Sanford today, he told prosecutor Bernie de la Rionda he opened the sliding door to his patio and stepped outside. He saw two people struggling and one soon was on top of the other in a straddling position. He said he could tell it was serious.

JOHN GOOD: Because it looked like there were strikes being thrown or punches being thrown, but as I clarified, due to the lighting, it could have also been, you know, holding down.

ALLEN: This is a story John Good has told many times before. He spoke to police that night and, over the next 16 months, was interviewed by other police, state investigators, prosecutors and the defense. In an early interview, he said the person on top was, quote, "raining down blows in a style similar to MMA," Mixed Martial Arts fighting. In more recent depositions, though, Good modified that description, saying he's not sure now he saw any blows struck.

In the courtroom, he said he never saw Trayvon Martin slam George Zimmerman's head against the concrete as the defendant claims. De la Rionda picked up the questioning.

BERNIE DE LA RIONDA: The person on top - could you tell where that person on top was actually striking? And here's what I'm going to do.

(SOUNDBITE OF SLAPPING NOISE)

RIONDA: Were they going like this?

GOOD: I could not hear that. No.

RIONDA: Could you hear...

(SOUNDBITE OF POUNDING NOISE)

RIONDA: ...this?

GOOD: No.

ALLEN: Several other neighbors from the townhome community who have testified so far have raised questions about parts of Zimmerman's story. Except for John Good, though, none actually saw the fight before the shot was fired.

Under questioning by Zimmerman's lawyer Mark O'Mara today, Good confirmed some key parts of Zimmerman's version of events.

MARK O'MARA: The person who you now know to be Trayvon Martin was on top. Correct?

GOOD: Correct.

O'MARA: And he was the one who was raining blows down on the person on the bottom, George Zimmerman. Right?

GOOD: That's what it looked like.

ALLEN: O'Mara also got Good to confirm another key part of Zimmerman's story that the Neighborhood Watch volunteer was the one calling for help.

O'MARA: You now believe that that was George Zimmerman's voice. Correct?

GOOD: I never said that.

O'MARA: Do you believe...

GOOD: It could have been his, but I was not 100 percent sure.

O'MARA: I'm not asking for 100 percent certainty. I'm asking you to use your common sense and to tell us if you think that that was George Zimmerman's voice screaming for help, the person on the bottom.

GOOD: That's just my opinion.

ALLEN: Today caps the first week of testimony in the trial. The defense expects to begin its case next week. Under Florida's Stand Your Ground Law because Zimmerman claims he acted in self-defense, there's a presumption of innocence. To convict him of murder, prosecutors have to convince the six-person jury that he shot Trayvon Martin, not to protect himself, but because he wanted to.